A load exhibits resistive impedance, inductive impedance, capacitive impedance or a combination thereof toward a power supply device. The power factor approaches 1, when the current input to the load is in phase with the voltage applied to the load. When the power factor is less than 1, the power transmitted may suffer a loss because of phase mismatch between the current and voltage or the introduction of noise. According to the prior art, to convert an AC power of a utility grid into a DC voltage, large-capacity capacitors are connected in parallel to reduce the second-order ripple component of a DC-link (DC bus) voltage and thus provide a stable DC-link voltage to a succeeding voltage transformer. However, the current distortion of the aforesaid circuit is large and contains plenty of harmonic components, thereby reducing the power factor. To reduce noise and enhance efficiency, the power supply device is usually equipped with an active power factor correction (PFC) circuit. The active power factor correction circuit uses the high-frequency switching of a power switch to cause the AC input current to follow the AC input voltage in order to attain an input current which approximates to a sinusoidal waveform and is in phase, so as to increase the power factor and reduce current harmonics.
A conventional boost PFC circuit is controlled with double loops, wherein a current control loop ensures that the input impedance of an inverter becomes resistive, whereas a voltage control loop regulates the output voltage. Since a power factor depends on the 120 Hz ripple of the output voltage, the compensation for a voltage loop not only takes into account the stability of the output voltage but also addresses the need to reduce the harmonic distortion which might otherwise be caused by the output voltage ripple. To prevent input current distortion, a conventional method involves setting the bandwidth of a voltage loop to 20 Hz approximately or even less than 20 Hz. However, the method has a drawback, that is, the system manifests slower transient response to a change of the load. To speed up the system's transient response to a change of the load, it is necessary to increase the bandwidth of the voltage loop at the expense of the fidelity of the waveform of the input current. As a result, persons skilled in the art are in a dilemma between a slow transient response from the system and a distorted waveform of the input current.